How do the H.264 encoding settings work in blender?
Further down the page you refer to you, under additional information you will find ABR (Average Bit Rate) where you will find the -bitrate, -minrate, -maxrate and -bufsize that correspond to blender's settings.

The bitrate is the target rate (average over the length of video) that you want to achieve. The minimum and maximum define how far from the average it is allowed to deviate.

Video compression works by only updating areas of video that change and compressing those areas. Like jpeg the amount of compression can vary depending on the quality you want in the final image. If high quality compression pushes the bitrate higher than the maximum limit then it needs to lower the quality to stay in the range.